Behavioral performance in many perceptual tasks is enhanced when a cue directs the subject's spatial attention to the relevant location in the visual field. The neuronal basis of this improvement in performance has been studied in the macaque monkey, where the activity of neurons in several visual areas has been shown to increase when a task directs attention to stimuli within a neuron's receptive field. We recently found, using functional magnetic resonance imaging (fMRI), that spatial attention also modulates neuronal activity in the earliest levels of visual processing of the human brain. Our long-term objective is to obtain a detailed understanding of where and how spatial attention modulates neuronal responses in the human visual cortex (and the lateral geniculate nucleus), and to examine how these attentional modulations of neuronal response improve behavioral performance. We will address four questions: (1) What visual areas in the human visual system are affected by spatial attention? (2) How does attending to different features of a stimulus (e.g. speed, color, and shape) influence responses in the human brain? (3) Does spatial attention modulate the baseline responses of neurons, or does it affect the stimulus-driven response? (4) Can the modulation of neuronal responses by spatial attention in early visual areas predict changes in behavioral performance of an observer? We will address these questions using a combination of new quantitative fMRI techniques, visual psychophysics, and computational modeling. Behavioral deficits in the ability to allocate spatial attention have been found in certain clinical subpopulations including people with developmental dyslexia and attentional deficit hyperactivity disorder (ADHD). Our proposed studies will therefore provide insight into the neurological basis of these disorders.